Royal Mail said: ‘We cannot yet determine the amount or range of potential loss; however, it is possible that it could be material.’

Investigation: Five companies Royal Mail, Dutch rival TNT Express, US goliath FedEx, Deutsche Post and La Poste have already been drawn into the alleged scandal

TNT Express has around a fifth of the French market, while state-owned La Poste represents the vast majority of the remainder.

Royal Mail has been banking on strong parcels growth, in the UK and internationally, to offset the decline in traditional letter post.

Shares, which have fallen by more than 20 per cent since peaking above £6 earlier this year, slipped 2 per cent to finish 9.2p lower at 479.5p.

Analysts were split over the potential severity of the fallout for Royal Mail.

Alex Paterson at Espirito Santo said: ‘French Antitrust law permits a maximum fine of 10 per cent of worldwide turnover. ‘This would lead to a worst case scenario of around a £160million fine for RMG (10 per cent of GLS revenue).’

But Robin Byde, analyst at Cantor Fitzgerald, said it was unlikely the fines would be too severe.

‘What I’ve seen in the past is that, given this is a small division, it might be quite noisy but the fines will probably be quite minor.’